Ultra-light sound insulator

ABSTRACT

A sound insulator of the invention includes a sound absorption layer and an air-impermeable resonance layer, which are bonded to each other via an adhesive layer. The sound absorption layer has a thickness in a range of 5 to 50 mm, an area-weight of not greater than 2000 g/m 2  and a two-layer structure of a high-density sound absorption layer and a low-density sound absorption layer. The high-density sound absorption layer is bonded to the air-impermeable resonance layer via the adhesive layer and has a density in a range of 0.05 to 0.20 g/cm 3  and a thickness in a range of 2 to 30 mm. The low-density sound absorption layer is bonded to the other face of the high-density sound absorption layer via an adhesive layer and has a density in a range of 0.01 to 0.10 g/cm 3  and a thickness in a range of 2 to 30 mm.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 13/443,028 now U.S. Pat. No. 8,637,145, filed Apr. 10, 2012, whichis a divisional application of U.S. application Ser. No. 10/551,124,filed Sep. 26, 2005, now U.S. Pat. No. 8,158,246 issued Apr. 17, 2012,which is a National Stage Entry of international application no.PCT/JP2004/003902, filed Mar. 23, 2004, the contents of each of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ultra-light sound insulator thatprevents propagation of noise and other undesired sound from an engineroom or any other vehicle exterior into a vehicle interior. Morespecifically the invention pertains to an ultra-light sound insulatorthat is extremely light in weight and effectively absorbs noise andother undesired sound to prevent their propagation into the vehicleinterior.

2. Description of the Related Art

Patent Document 1 discloses a multifunctional kit (41), which is used invehicles to attain noise reduction and heat insulation and morespecifically to have sound-absorbing, sound-insulating,oscillation-damping, and heat-insulating effects on floor insulation,end wall insulation, door covering, and roof inner covering. Themultifunctional kit (41) includes at least one areal vehicle part (11)and a multi-layer noise-reducing assembly package (42). The assemblypackage (42) has at least one porous spring layer (13), which ispreferably formed from an open-pored foam layer. An air gap (25) isinterposed between the assembly package (42) and the areal vehicle part(11). The multi-layer assembly package (42) does not have a heavy-weightlayer to give the ultra-light kit (41) suitable for the optimumcombination of sound-insulating, sound-absorbing, andoscillation-damping properties. The assembly package (42) also has amicro-porous stiffening layer (14), which preferably consists of anopen-pored fiber layer or fiber/foam composite layer. The micro-porousstiffening layer (14) has a total airflow resistance of R_(t)=500 Nsm⁻³to R_(t)=2500 Nsm⁻³, in particular of R_(t)=900 Nsm⁻³ to R_(t)=2000Nsm⁻³, and an area-weight (weight per unit area) of m_(F)=0.3 kg/m² tom_(F)=2.0 kg/m², in particular of m_(F)=0.5 kg/m² to m_(F)=1.6 kg/m².The advantages of this kit are particularly evident with the applicationof thin steel sheeting, light aluminum sheeting, or organo-sheeting, asis favorably used today in the automobile industry. A further advantageof this kit lies in the extremely low heat conductivity of the appliedporous spring layer, which leads to the fact that this kit apart fromits good acoustic effectiveness (sound insulation effects) also has goodheat insulation.

Patent Document 2 discloses a sound insulator 10 for vehicles. In thisprior art sound insulator 10, a first air-permeable sound absorptionlayer 20, an air-impermeable sound insulation layer 30, and a secondair-permeable sound absorption layer 40 are arranged in this order froma vehicle interior 100. The first air-permeable sound absorption layer20 does not have an air-impermeable layer on the side of the vehicleinterior, while the second air-permeable sound absorption layer 40 doesnot have an air-impermeable layer on the opposite side of the vehicleinterior. The sound insulator is light in weight and is designed toeffectively reabsorb noise, which has passed through the sound insulatorand leaked into the vehicle interior, and to effectively absorb noiseincoming from a site other than an engine room into the vehicleinterior.

Patent Document 3 discloses an automobile insulator (20) attached to thevehicle interior side of a vehicle body panel (10). The insulator (20)has a mono-layer sound absorption layer (21), the base of which is afiber molded object. The insulator (20) is constructed as anair-permeable insulator to absorb the noise, which is propagated throughthe vehicle body panel (10) and enters the sound absorption layer (21),while absorbing reflected noise, which is transmitted through the soundabsorption layer (21), is reflected from the inner face of a vehicleinterior panel (40), and enters again the sound absorption layer (21)from the surface side. A surface layer (22) of high-density fibers setto have a higher density than the area density of the sound absorptionlayer (21) is formed on at least one of the surface and the rear face ofthe sound absorption layer (21). A surface layer (27) of a foamed resinsheet material is also formed to wholly or partly cover at least one ofthe surface and the rear face of the sound absorption layer (21). Thisstructure excludes the conventional sound insulation layer to reduce theweight, while preventing an increase in sound pressure in the instrumentpanel (40) to enhance the stillness in the vehicle interior.

Patent Document 4 discloses a laminated object obtained by integrallyforming a polyolefin resin foam having a skin peel strength of notgreater than 20 N/cm and an L value of not higher than 60 and a bulkynon-woven fabric having a thickness of not less than 5 mm and a densityof not higher than 50 kg/cm³. The laminated object has an area-weight ofnot greater than 3 kg/m². The laminated object is light in weight andeasily shaped and has high recyclability and good appearance.

A dash silencer including a surface layer and a sound absorption layer(Patent Document 1+Patent Document 3) has been proposed by takingadvantage of the air-flow resistance.

The transmission loss and the sound absorption power of the conventionalsound insulation structure are compared with those of the structuredisclosed in Patent Document 1. In this discussion, a low frequencydomain includes 315 Hz and lower as the ⅓ octave band center frequency.A medium frequency domain includes 400 to 1600 Hz. A high frequencydomain includes 2000 Hz and higher.

The transmission loss and the sound absorption power of the conventionalsound insulation type structure (see FIG. 27, hereafter referred to asthe ‘structure of FIG. 27’) are compared with those of the structuredisclosed in Patent Document 1 (see FIG. 28, hereafter referred to asthe ‘structure of FIG. 28’).

The dash silencer having the structure of FIG. 27 has an area-weight of6.0 kg/m², whereas the structure of FIG. 28 has a currently availableeffective area-weight of 2.0 kg/m². These products are attached to avehicle body panel, which has an area-weight of 6.2 kg/m².

According to the transmission loss curve of FIG. 29( a), the structureof FIG. 27 has the greater transmission loss than the weight law. Thisis ascribed to the double-wall structure of the air-impermeable surfacelayer and the panel and the presence of the intermediate soundabsorption material having the air-flow resistance. The high area-weightof rubber sheet, however, causes a significant transmission resonance ina low frequency domain to drastically lower the transmission loss.

According to the transmission loss curve of FIG. 29( a), the structureof FIG. 28 has the smaller transmission loss than the weight law. Thestructure of FIG. 28 has also a double-wall structure of theair-permeable surface layer and the panel, however, the surface layerlets air through, and this causes sound leakage in a high frequencydomain. The structure of FIG. 28 accordingly does not give sufficienttransmission loss for sound insulation.

According to the sound absorption rate curve of FIG. 29( b), thestructure of FIG. 27 has a peak of sound absorption rate, due to strongsurface resonance, in a low frequency domain, while only little orsubstantially no sound absorption rate in the medium to high frequencydomain.

According to the sound absorption rate curve of FIG. 29( b), thestructure of FIG. 28 takes advantage of the surface resonance of thesurface layer having the high air-flow resistance and the soundabsorbing power of the rear sound absorption layer to attain soundabsorption power in the medium to high frequency domain.

The indirect noise, which incomes from everywhere of the automobile andis reflected, rather than the direct noise, which directly incomes fromthe dash panel to the dash silencer, more significantly affects theactual stillness in the vehicle interior. The structure of PatentDocument 1 has significantly lowered transmission loss but relativelyhigher sound absorbing power in a medium to high frequency domain, thusattaining somewhat equivalent stillness in the vehicle interior to theeffects of the conventional structure. The structure of Patent Document1, however, has an advantage of significant weight reduction of aproduct and has favorably been applied to the recent dash panelstructure.

[Patent Document 1] Patent Publication Gazette No. 2000-516175

[Patent Document 2] Patent Laid-Open Gazette No. 2001-347899

[Patent Document 3] Patent Laid-Open Gazette No. 2002-220009

[Patent Document 4] Patent Laid-Open Gazette No. 2002-347194

The automobile of some vehicle structure has large effects of directnoise. The structure of FIG. 28 may give an insufficient transmissionloss (see FIG. 29( a)) and thus fail to attain the required stillness inthe vehicle interior. Additionally, actual products have designedpatterns and varying thickness of the sound absorption layer in a rangeof 1 to 30 mm. The dash silencer having the structure of FIG. 28disclosed in Patent Document 1 takes advantage of the sound absorbingpower of the sound absorption layer in the high frequency domain.Reduction in thickness of the sound absorption layer thus results in thelowered sound absorbing power. Additionally, the sound absorption layeris made of felt having a thickness in a range of 30 to 50 mm. The thinwall portion has the lowered air-flow resistance than the other wallportion and does not give the sufficient sound absorbing power. The dashsilencer having the structure disclosed in Patent Document 1 thatassures the stillness in the vehicle interior due to sound absorbingpower may thus not exert sufficient performances.

The prior art sound insulator is designed to reduce the noise directlyincoming from the vehicle exterior and have good sound absorbing powerin a wide frequency domain, while not having the sufficientcountermeasure to absorb reflected noise in the vehicle interior. Asshown in FIG. 30, a ⅓ octave band center frequency domain of 800 to 1600Hz is essential for the clearness of conversion. The prior art structurehas insufficient sound absorption effects at the frequency of about 1000Hz, which is important for cleanness of conversation.

In the sound insulator of Patent Document 2, the sound absorbing powerof the sound absorption material is used to absorb sound in a frequencydomain of not lower than 1000 Hz, as shown in FIG. 31. The reducedthickness of the sound absorption layer thus tends to lower the soundabsorption rate.

The sound insulator having the structure of FIG. 28 functions to absorbreflected sound in the vehicle interior, but there is no clear method ofregulating the sound absorption frequency.

The prior art sound insulators disclosed in Patent Documents 3 and 4have not given any consideration to the effects of the restricting stateat the interface between the sound absorption layer and the surfacelayer and the air permeation of the surface layer on the soundabsorption characteristics and the sound insulation characteristics.Actual products have complicated shapes and require the interfacialadhesion strength. The prior art sound insulators may thus havedifferent sound absorption and sound insulation characteristics fromdesigned conditions and may not be usable in limited spaces.

SUMMARY OF THE INVENTION

The object of the invention is thus to enhance sound insulation fromdirect noise incoming from a vehicle panel and more specifically toraise transmission loss in a medium to high frequency domain generallyhaving less transmission loss. The object of the invention is also toensure sufficient sound absorption even in a thin-wall sound absorptionlayer of an actual odd-shaped product and more specifically to enhancesound absorbing power in the medium frequency domain (including a noiselevel of a voice-tone frequency domain) to high frequency domain. Theobject of the invention is further to enhance sound absorbing power in afrequency domain of 315 to 800 Hz generally having poor soundabsorption. The object of the invention is also to reduce the weight ofa sound insulator.

In order to attain the above and the other related objects, the inventorof this invention has found the optimum conditions of adhesion at aninterface between a sound absorption layer and an air-impermeableresonance layer and has significantly reduced the weight of theair-impermeable resonance layer. The transmission loss and the soundabsorbing rate in the relevant frequency domain are regulated in orderto ensure sufficient sound insulation from noise incoming from a vehicleexterior and sufficient sound absorption in the vehicle interior, andthereby improve the stillness in the vehicle interior.

The invention recited in claim 1 is thus directed to an ultra-lightsound insulator, which includes: a sound absorption layer that is lightin weight and has a thickness in a range of 1 to 100 mm, a density in arange of 0.01 to 0.2 g/cm³ or more preferably in a range of 0.03 to 0.08g/cm³; and an air-impermeable resonance layer that is bonded to thesound absorption layer via an adhesive layer and has an area-weight(weight per unit area) of not greater than 600 g/m² or more preferablyof not greater than 300 g/m². An adhesion strength of the adhesive layeragainst the sound absorption layer and the air-impermeable resonancelayer is set in a range of 1 to 20 N/25 mm or more preferably in a rangeof 3 to 10 N/25 mm under conditions of a peel angle of 180 degrees and apeel width of 25 mm. An adhesion area of the adhesive layer is 50 to100% or more preferably 80 to 100% of a whole interface between thesound absorption layer and the air-impermeable resonance layer. Thesound absorption layer faces to a vehicle body panel, while theair-impermeable resonance layer faces to a vehicle interior.

The peeling method is in conformity with ‘JIS K6854, FIG. 4: 180-degreepeel’ and adopts a peeling rate of 200 mm/minute.

The air-impermeable resonance layer and the sound absorption are bondedto each other with a sufficient adhesion force by means of the adhesivelayer at the interface. The sound insulator of the invention takesadvantage of the resonance of the sound absorption layer and theair-impermeable resonance layer at the interface to ensure sufficientsound absorption. The air permeability is measured with a Frazil-typeair permeability tester in conformity with JIS L1018 8.3.3.1 concerningair permeability of knitted fabrics or an equivalent air permeabilitytester having extremely high correlativity. The material is determinedas air impermeable when the measurement result is not greater than 0.1cm³/cm²·sec, which is the lowest measurable limit. The sound absorptionlayer preferably includes an air layer.

The inventor of the present invention has completed the invention, basedon the finding that the peel strength and the adhesion area of theadhesive layer that indicate the state of the interface between theair-impermeable resonance layer and the sound absorption layer affectsthe sound absorbing power. The ultra-light sound insulator of theinvention takes advantage of the resonance at the interface between theair-impermeable resonance layer and the sound absorption layer toenhance the sound absorbing power. The presence of the adhesive layerinterposed between the air-impermeable resonance layer and the soundabsorption layer effectively regulates the frequency of sound absorbedat the interface. The sound in the vehicle interior is absorbed bymembrane resonance of the air-impermeable resonance layer and the soundabsorption layer.

The air-impermeable resonance layer may be formed over the whole face orpartial face of the sound absorption layer or may be formed on eitherthe surface or the rear face of the sound absorption layer.

The structure of the ultra-light sound insulator includes the soundabsorption layer and the air-impermeable resonance layer (for example,an air-impermeable thin film layer or an ultra-light air-impermeablefoam layer), which faces to the vehicle interior. The sound absorptionlayer and the adhesive layer may be air impermeable or air permeable.The sound absorption layer may be made of an air-permeable material oran air-impermeable material, as long as the material ensures sufficientsound absorbing power. For example, both air-permeable andair-impermeable polyurethane molds are applicable to the material of thesound absorption layer.

The adhesion area of the adhesive layer is 50 to 100% or preferably notless than 80% of the whole interface between the air-impermeableresonance layer and the sound absorption layer. The adhesion area maycover the whole interface or the partial interface. It is desirable thatthe sound absorption layer is continuously bonded to the air-impermeableresonance layer via the adhesive layer. Dot adhesion at a density of 1to 50 dots/cm² or thread adhesion may be adopted. An adhesive film mayalso be applied for adhesion of the whole interface.

The adhesion strength is set in a range of 1 to 20 N/25 mm or morepreferably in a range of 3 to 10 N/25 mm under the conditions of a peelangle of 180 degrees and a peel width of 25 mm.

The air-impermeable resonance layer is made of an air-impermeablematerial, for example, a resin foam or a resin film. The soundabsorption layer is made of either an air-impermeable material or anair-permeable material, for example, a thermoplastic felt of reusedsynthetic fibers or PET fibers with binder fibers. The adhesive layer ismade of either an air-impermeable material or an air-permeable material,for example, ethylene vinyl acetate copolymer (EVA) or polyurethaneadhesive.

The sound absorption layer of the invention recited in claim 2 is anultra-light sound insulator that has a multi-layer structure of ahigh-density sound absorption layer and a low-density sound absorptionlayer.

An invention recited in claim 3 is an ultra-light sound insulator inaccordance with claim 2, wherein the high-density sound absorption layerhas a density in a range of 0.05 to 0.20 g/cm³ and a thickness in arange of 2 to 70 mm, and the low-density sound absorption layer has adensity in a range of 0.01 to 0.10 g/cm³ and a thickness in a range of 2to 70 mm.

An invention recited in claim 4 is an ultra-light sound insulator inaccordance with either one of claim 2 and 3, wherein the high-densitysound absorption layer has an initial compression repulsive force in arange of 30 to 600 N or more preferably in a range of 50 to 300 N, andthe low-density sound absorption layer has an initial compressionrepulsive force in a range of 5 to 300 N or more preferably in a rangeof 10 to 100N and the initial compression repulsive force of thehigh-density sound absorption layer is at least 1.2 to 40 times theinitial compression repulsive force of the low-density sound absorptionlayer and the high-density sound absorption layer has a thicknessoccupying 20 to 80% of the thickness of the sound absorption layer, morepreferably the initial compression repulsive force of the high-densitysound absorption layer is 1.5 to 5 times the initial compressionrepulsive force of the low-density sound absorption layer and thehigh-density sound absorption layer has a thickness occupying 40 to 60%of the thickness of the sound absorption layer.

The compression initial repulsive force and the thickness of thehigh-density sound absorption layer affect the spring in a spring-massvibration system. Adhesion of the high-density sound absorption layerhaving the higher initial repulsive force to the air-impermeableresonance layer via the adhesive layer enhances the rigidity of theair-impermeable resonance layer and shifts the resonance frequency tothe higher frequency. The high-density sound absorption layer and thelow-density sound absorption layer are required to have an adequatedifference of rigidity to induce the resonances in desired highfrequency domain and low frequency domain.

Each sound absorption material used for the sound absorption layer istrimmed to a cylindrical sample of 100 mm φ and 20 mm in thickness formeasurement of the initial compression repulsive force.

FIG. 1 shows a method of measuring the initial compression repulsiveforce, where a load is applied to compress a cylindrical sample (100 mmφ) of each sound absorption material.

As shown in FIG. 1, a load is applied onto the top face of the sample,and the repulsive force under the condition of a compression to a depthof 5 mm is measured with a load tester like a Tensilon tester. Theloading speed is 50 mm/minute. For the purpose of reference, therepulsive force is measured under the conditions of a compression to adepth of 2.5 mm and a compression to a depth of 7.5 mm.

FIG. 2 is a table showing results of measurement of the initialcompression repulsive force with regard to PET (polyethyleneterephtalate) felt, RSPP (reused sound insulating material made fromshredder dust), and PUF (polyurethane foam). The compression repulsiveforce of the sound absorption layer is related to the elastic modulus ofa damping material. The felt conventionally used as a kind of soundinsulating materials is a kind of damping materials. The dampingmaterial absorbs vibration energy and converts the absorbed vibrationenergy into thermal energy. A loss coefficient η is a factor showing thedamping effect. The loss coefficient η is expressed by Equation 1 givenbelow:

$\begin{matrix}{{\eta = {\eta^{\prime} \times \frac{E\; 2}{E\; 1} \times \left( \frac{h\; 2}{h\; 1} \right)^{2}}}\begin{matrix}{\eta\text{:}\mspace{14mu}{Loss}\mspace{14mu}{coefficient}} \\{\eta^{\prime}\text{:}\mspace{14mu}\begin{matrix}{{Loss}\mspace{14mu}{coefficient}\mspace{14mu}{of}\mspace{14mu}{sound}} \\{{absorption}\mspace{14mu}{layer}}\end{matrix}} \\{E\; 1\text{:}\mspace{14mu}\begin{matrix}{{Elastic}\mspace{14mu}{modulus}\mspace{14mu}{of}} \\{{resonance}\mspace{14mu}{layer}}\end{matrix}} \\{E\; 2\text{:}\mspace{14mu}\begin{matrix}{{Elastic}\mspace{14mu}{modulus}\mspace{14mu}{of}} \\{{sound}\mspace{14mu}{absorption}\mspace{14mu}{layer}}\end{matrix}} \\{h\; 1\text{:}\mspace{14mu}\begin{matrix}{{Thickness}\mspace{14mu}{of}\mspace{14mu}{resonance}} \\{layer}\end{matrix}} \\{h\; 2\text{:}\mspace{14mu}\begin{matrix}{{Thickness}\mspace{14mu}{of}\mspace{14mu}{sound}} \\{{absorption}\mspace{14mu}{layer}}\end{matrix}}\end{matrix}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

It is preferable that the sound absorption layer has a two-layerstructure including a high-density sound absorption layer and alow-density sound absorption layer of different materials. Anotherpreferable example of the sound absorption layer is made of a singlematerial having a density gradient from a higher density to a lowerdensity.

Preferably, the sound absorption layer made of the two-layer structureof different materials is a combination of a higher-density soundabsorption material with a lower-density sound absorption material. Themono-layer structure having the density gradient from a higher densityto a lower density exerts the similar effects to those of the two-layerstructure, when its higher density side is bonded to the air-impermeableresonance layer via the adhesive layer.

For example, one face of the high-density sound absorption layer may bebonded to the air-impermeable resonance layer via the adhesive layer,while one face of the low-density sound absorption layer is bonded viaanother adhesive layer to or laid on the other face of the high-densitysound absorption layer, which is opposite to the air-impermeableresonance layer. For another example, mono-layer structure having thedensity gradient from a higher density to a lower density may also beused.

Preferable materials for the sound absorption layer includethermoplastic felts, polyester felts like PET (polyethyleneterephtalate) felt, polyurethane molds, polyurethane foam slubs, andRSPP.

An invention recited in claim 5 is an ultra-light sound insulator inaccordance with claim 1, wherein the sound absorption layer has amono-layer structure and has a density in a range of 0.02 to 0.20 g/cm³and a thickness in a range of 2 to 70 mm. It is preferable that thesound absorption layer is made of a single material.

An invention recited in claim 6 is an ultra-light sound insulator inaccordance with claim 5, wherein the sound absorption layer has aninitial compression repulsive force in a range of 2 to 200 N or morepreferably in a range of 20 to 100 N.

An invention recited in claim 7 is an ultra-light sound insulator inaccordance with any one of the claims 1 through 6, wherein a secondsound absorption layer is bonded to the other face of theair-impermeable resonance layer, which is not in contact with theadhesive layer but faces to the vehicle interior, and the second soundabsorption layer has a density in a range of 0.01 to 0.2 g/cm³ or morepreferably in a range of 0.05 to 0.15 g/cm³ and a thickness in a rangeof 1 to 20 mm or more preferably in a range of 4 to 10 mm.

Any method may be adopted to fix the second sound absorption layer tothe air-impermeable resonance layer. One method does not use anyadhesive but simply lays the second sound absorption layer on theair-impermeable resonance layer. For example, the second soundabsorption layer, the resonance layer, and the sound absorption layerare fixed via a fastener (not shown) to a vehicle body panel like a dashpanel or a floor panel. Another method is local adhesion like dotadhesion at a pitch of 20 to 100 mm. Still another method is overalladhesion via an adhesive layer. The adhesion strength of the secondsound absorption layer to the air-impermeable resonance layer is in arange of 0.1 to 20 N/25 mm or more preferably in a range of 3 to 10 N/25mm under the conditions of a peel angle of 180 degrees and a peel widthof 25 mm. The second sound absorption layer may be formed to cover overthe whole single face of the air-impermeable resonance layer, or may beformed only at a site of high noise reflection in the vehicle interioraccording to the requirements. The second sound absorption layer mayhave a mono-layer structure or a multi-layer structure. The multiplelayers of the second sound absorption layer may be joined together byadhesion. Multiple sound absorption layers may be joined by an adhesiveagent, an adhesive film, or by mechanical bonding, such as mechanicalneedle punching force.

An invention recited in claim 8 is an ultra-light sound insulator inaccordance with claim 7, wherein the second sound absorption layer haseither of a mono-layer structure and a multi-layer structure.

An invention recited in claim 9 is an ultra-light sound insulator inaccordance with either one of claim 7 and 8, wherein the second soundabsorption layer has a multi-layer structure of a lower layer and anupper layer. The lower layer of the second sound absorption layer may bebonded to the air-impermeable resonance layer or otherwise the upperlayer and the lower layer of the second sound absorption layer may belaid one upon the other by means of a mechanical boring force. Moreprecisely, it is preferable that a lower layer of the second soundabsorption layer is bonded to a film resonance layer or an upper filmlayer and a lower felt layer are laid one upon the other by needlepunching.

An invention recited in claim 10 is an ultra-light sound insulator inaccordance with any one of claims 1 through 9, wherein theair-impermeable resonance layer is either of a foam and a film, and theair-impermeable resonance layer has a thickness in a range of 1 to 7 mmor more preferably in a range of 2 to 3 mm in the case of the foam,while having a thickness in a range of 10 to 600 μm or more preferablyin a range of 20 to 300 μm in the case of the film.

The sound absorption layer is made of a low-density air-impermeable orair-permeable material and has sound absorbing power. Theair-impermeable resonance layer is required to be sufficiently light inweight for the resonance at a low pitch or with low vibration energy.

Preferable materials for the air-impermeable resonance film layerinclude olefin resin films, polyester films like polyethyleneterephthalate (PET) film, polyurethane resin films, and theircombinations. Preferable materials for the independent air-impermeableresonance foam include olefin foams like polypropylene foam (PPF) andpolyethylene foam (PEF).

The ultra-light sound insulator of the invention has the especially goodsound absorbing power in the frequency band of 1000 to 1600 Hz for theimproved clarity of conversation. This is ascribed to a continuous,arbitrary variation in thickness of the sound absorption layer. Thestructure of the invention effectively improves sound absorption by thesheet resonance in this frequency band, thus ensuring the favorablestillness in the vehicle interior. The ultra-light sound insulator ofthe invention has the less thickness but takes advantage of the sheetresonance to ensure the high sound absorption rate.

The structure of the invention significantly reduces the weight of theair-impermeable resonance layer, compared with the prior art soundabsorber. In the ultra-light sound insulator of the invention, theair-impermeable resonance layer has an area-weight of not greater than600 g/m² or more preferably of not greater than 300 g/m². Theair-impermeable resonance layer is either a foam or a film and has athickness in a range of 1 to 7 mm or more preferably in a range of 2 to3 mm in the case of the foam, while having a thickness in a range of 10to 600 μm or more preferably in a range of 20 to 300 μm in the case ofthe film.

The conventional sound insulator has an area-weight in a range of 4000to 10000 g/m², and the conventional sound absorber has an area-weight ina range of 500 to 2000 g/m². In the ultra-light sound insulator of theinvention, on the other hand, the air-impermeable resonance layer has anarea-weight of not greater than 200 g/m².

The adhesive layer has a thickness in a range of 1 to 100 μm or morepreferably in a range of 5 to 50 μm and an area-weight in a range of 5to 200 g/m² or more preferably in a range of 10 to 100 g/m². Theadhesive layer may have any arbitrary density.

The terminology ‘whole interface’ means the whole interface where theair-impermeable resonance layer and the sound absorption layer can bejoined by adhesion. Here S1 and S2 denote the area of theair-impermeable resonance layer and the sound absorption layer. In thecase of S1=S2, the area of the whole interface S=S1=S2. In the case ofS1>S2, the area of the whole interface S=S2. In the case of S1<S2, thearea of the whole interface S=S1. The terminology ‘peel’ means releaseof the sound absorption layer from the bonded air-impermeable resonancelayer under preset measurement conditions. The peeling state mayrepresent surface destruction of the material (for example, surfacedestruction of the felt), interfacial release of the adhesive (forexample, release with the adhesive entirely left on the sound absorptionlayer), cohesive release of the adhesive (for example, release with theadhesive in the threading state left on both the sound absorption layerand the air-impermeable resonance layer), or a combination of surfacedestruction of the material, interfacial release of the adhesive, andcohesive release of the adhesive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a method of measuring initial compression repulsive force;

FIG. 2 is a table showing results of measurement of the initialcompression repulsive force;

FIG. 3 shows the basic structure of a dash silencer 1 in a firstembodiment of the invention;

FIG. 4 is a sectional view showing a dash panel with the dash silencer 1attached thereto;

FIGS. 5( a) and 5(b) are graphs respectively showingfrequency-transmission loss curves and frequency-sound absorption ratecurves with regard to the dash silencer 1 of the first embodiment andthe prior art structures of FIGS. 27 and 28;

FIG. 6( a) is a graph showing frequency-sound absorption rate curveswith regard to the dash silencer 1 of the first embodiment;

FIG. 6( b) is a graph showing frequency-sound absorption rate curveswith regard to the dash silencer without an air-impermeable resonancelayer;

FIGS. 7( a) and 7(b) are graphs respectively showingfrequency-transmission loss curves and frequency-sound absorption ratecurves under sufficient adhesive conditions and under insufficientadhesive conditions in the dash silencer 1 of the first embodiment;

FIG. 8 is a graph showing ⅓ octave band frequency-transmission losscharacteristic curves with regard to the dash silencer 1;

FIG. 9 is a graph showing ⅓ octave band frequency-sound absorption ratecharacteristic curves with regard to the dash silencer 1;

FIG. 10( a) shows the basic structure of a dash silencer 201 (having avarying-density, two-layer sound absorption layer) in a secondembodiment of the invention;

FIG. 10( b) shows the basic structure of a dash silencer 301 (having asecond sound absorption layer bonded to an air-impermeable resonancelayer) in a third embodiment of the invention;

FIG. 11 is a graph showing frequency-transmission loss curves withregard to a varying-density, two-layer sound absorption layer and amono-layer sound absorption layer on a fixed density in the dashsilencer 201 of the second embodiment;

FIG. 12 is a graph showing frequency-sound absorption rate curves withregard to a varying-density, two-layer sound absorption layer and amono-layer sound absorption layer of a fixed density in the dashsilencer 201 of the second embodiment;

FIG. 13 is a graph showing frequency-sound absorption rate curves whenthere is an adhesive layer and the sound absorption layer has a densityvariety.

FIG. 14 is a graph showing frequency-transmission loss curves in thepresence and in the absence of the second sound absorption layer undervarious adhesive conditions;

FIG. 15 is a graph showing the frequency-sound absorption rate curves inthe absence of the second sound absorption layer and under restrictionor non-restriction of the air-impermeable resonance layer with amaterial of no sound absorbing power in the dash silencer 301 of thethird embodiment;

FIG. 16 is a graph showing the frequency-sound absorption rate curvesunder restriction of the air-impermeable resonance layer with the secondsound absorption layer and with a material of no sound absorbing powerin the dash silencer 301 of the third embodiment;

FIG. 17 shows the basic structure of a dash silencer 401 (having amono-layer sound absorption layer) in a fourth embodiment of theinvention;

FIG. 18 is a graph showing frequency-transmission loss curves of thefourth embodiment;

FIG. 19 is a graph showing frequency-sound absorption rate curves of thefourth embodiment;

FIG. 20 shows the basic structure of a floor silencer 501 in a fifthembodiment;

FIGS. 21( a), 21(b), and 21(c) respectively show the basic structure ofComparative Example 1, the basic structure of Comparative Example 2, andthe basic structure of Example of the fifth embodiment;

FIG. 22( a) is a graph showing frequency-transmission loss curves withregard to the respective structures of FIGS. 21( a) through 21(c);

FIG. 22( b) is a graph showing frequency-sound absorption rate curveswith regard to the respective structures of FIGS. 21( a) through 21(c);

FIG. 23( a) is a graph showing frequency-transmission loss curves withand without a film in a modified structure of the fifth embodiment;

FIG. 23( b) is a graph showing frequency-sound absorption rate curveswith and without a film in the modified structure of the fifthembodiment;

FIG. 24 shows the basic structure of a floor silencer 601 in a sixthembodiment of the invention;

FIG. 25 is a plan view showing a measurement system of transmissionloss;

FIG. 26 is a plan view showing a measurement system of sound absorptionrate;

FIG. 27 shows a prior art sound insulation structure;

FIG. 28 shows a sound insulation structure disclosed in Patent Document1;

FIG. 29( a) is a graph showing frequency-transmission loss curves withregard to the prior art sound insulation structure shown in FIG. 27 andthe sound insulation structure of Patent Document 1 shown in FIG. 28

FIG. 29( b) is a graph showing frequency-sound absorption rate curveswith regard to the prior art sound insulation structure shown in FIG. 27and the sound insulation structure of Patent Document 1 shown in FIG. 28

FIG. 30 is a graph showing a variation in noise level in vehicleinterior; and

FIG. 31 is a graph showing a frequency-sound absorption rate curve inPatent Document 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The ultra-light sound insulator of the invention is discussed below asfirst through six embodiments with reference to the accompanyingdrawings.

First Embodiment

As shown in FIG. 3, a dash silencer 1 of a first embodiment has atwo-layer structure of a sound absorption layer 2 and an air-impermeableresonance layer 3. The sound absorption layer 2 has an air permeabilityin a range of 10 to 50 cm³/cm²·sec in the case of thermoplastic felt oran air permeability of not greater than 10 cm³/cm²·sec in the case ofpolyurethane foam. An adhesive layer 4 is interposed between the soundabsorption layer 2 and the air-impermeable resonance layer 3 for bondingthe two layers 2 and 3 to each other. The dash silencer 1 takesadvantage of resonance at the interface between the sound absorptionlayer 2 and the air-impermeable resonance layer 3 for sound absorption.

As shown in FIG. 4, an iron dash panel 10 parts a vehicle interior froma vehicle exterior (an engine room), and the dash silencer 1 of thefirst embodiment is formed along the inner surface of the vehicleinterior. The dash silencer 1 is designed to be ultra light in weightfor the enhanced fuel efficiency and the easy attachment but to stillhave sufficient sound absorption properties.

FIG. 4 shows the dash silencer 1 of the first embodiment. The vehicleinterior, the air-impermeable resonance layer 3, the adhesive layer 4,the sound absorption layer 2, the dash panel 10 as the vehicle body, andthe vehicle exterior are arranged in this order. The sound absorptionlayer 2 faces to the dash panel 10, whereas the air-impermeableresonance layer 3 faces to the vehicle interior. The sound absorptionlayer 2 is bonded to the dash panel 10. An augmentation material may beinterposed between them.

The sound absorption layer 2 is formed along the face of the dash panel10. The sound absorption layer 2 has an arbitrary varying thickness ofnot greater than 50 mm or more preferably in a range of 5 to 40 mm, anarea-weight (weight per unit area) in a range of 500 to 2000 g/m² ormore preferably in a range of 1000 to 1600 g/m², a density in a range of0.01 to 0.2 g/cm³ or more preferably in a range of 0.03 to 0.08 g/cm³,and an initial compression repulsive force in a range of 2 to 200 N ormore preferably in a range of 20 to 100 N. A compression molded part toa local thickness of 1 mm has an extremely high density of 0.5 g/cm³.This part lowers the sound absorbing power but ensures the soundinsulating power according to the weight law.

The sound absorption layer 2 is made of either an air-permeable materialor an air-impermeable material. A preferable material for the soundabsorption layer 2 is thermoplastic felt of reused synthetic fibers orPET fibers with binder fibers. One method of molding the soundabsorption layer 2 adds a low melting-point PET resin as a binder toregenerated PET fibers, aggregates the mixture on a conveyor belt in amat shape, heats and presses the mixture to a desired mat shape, heatsand softens the whole mat, and molds the softened mat to a desired shapealong the face of the dash panel 10 by a cold press metal mold having adesired mold shape. When a thermosetting resin is used as the binder,the fibers impregnated with the thermosetting resin are formed to adesired shape by hot pressing. The binder may be any of thermoplasticresins and thermosetting resins. The material and the molding method arenot restrictive, as long as the material is collective fibers havingexcellent sound absorbing properties.

As shown in FIG. 4, the sound absorption layer 2 arbitrarily varies itsthickness in the range of not greater than 50 mm, thereby varying thethickness of the dash silencer 1.

The randomly varying thickness of the sound absorption layer 2effectively absorbs sound in a wide frequency band of 315 to 4000 Hz asa whole.

The air-impermeable resonance layer 3 is formed on the sound absorptionlayer 2 and faces to the vehicle interior. The air-impermeable resonancelayer 3 absorbs sound in the vehicle interior mainly by the membraneresonance with the sound absorption layer 2. The air-impermeableresonance layer 3 is made of either air-impermeable resonance film orair-impermeable independent resonance foam. The air-impermeableresonance layer 3 has an area-weight of not greater than 200 g/m² ormore preferably of not greater than 100 g/m². The air-impermeableresonance layer 3 has a thickness in a range of 1 to 7 mm or morepreferably in a range of 2 to 3 mm in the case of the foam, while havinga thickness in a range of 10 to 200 μm or more preferably in a range of20 to 100 μm in the case of the film. The air-impermeable resonancelayer 3 has a density in a range of 0.02 to 0.1 g/cm³ or more preferablyin a range of 0.03 to 0.06 g/cm³ in the case of the foam, while having adensity in a range of 0.9 to 1.2 g/cm³ or more preferably in a range of0.9 to 1.0 g/cm³ in the case of the film.

Preferable materials for the air-impermeable resonance layer 3 includeolefin resin films, polyester films like polyethylene terephthalate(PET) film, polyurethane resin films, and their combinations. Preferablematerials for the air-impermeable resonance foam include olefin foamslike polypropylene foam (PPF) and polyethylene foam (PEF).

The adhesive layer 4 has an area-weight in a range of 5 to 200 g/m² ormore preferably in a range of 10 to 100 g/m² and a thickness in a rangeof 1 to 100 μm or more preferably in a range of 5 to 50 μm. The densityof the adhesive layer 4 is not restricted but may be equivalent to atypical value of adhesives. The adhesion strength of the adhesive layer4 is in a range of 1 to 20N/25 mm or more preferably in a range of 3 to10N/25 mm. The rate of the adhesion area is 50 to 100% or morepreferably 80 to 100%. The adhesion area may cover the whole interfaceor the partial interface. For example, the sound absorption layer 2 maycontinuously be bonded to the air-impermeable resonance layer 3 via theadhesive layer 4. Dot adhesion at a density of 1 to 50 dots/cm² orthread adhesion may be adopted. An adhesive film may be applied foradhesion of the whole interface. Available materials for the adhesivelayer 4 include ethylene vinyl acetate (EVA) resins, polyurethaneresins, chloroprene rubber (CR) latexes resins, styrene-butadienecopolymer rubbers (SBR) resins, acryl resins, and olefin resins. It is,however, not desirable to use any material having weaker adhesive forcethan a required level to sufficiently fix the air-impermeable resonancelayer 3 to the sound absorption layer 2.

Lamination by a carding machine or a random film making machine may beused for formation of the sound absorption layer 2 and theair-impermeable resonance layer 3. The adhesion plane of the soundabsorption layer 2 to the air-impermeable resonance layer 3 requiressmooth finishing. This assures the sufficient adhesion area andefficiently fixes the air-impermeable resonance layer 3 to the soundabsorption layer 2.

The improvement in sound insulation property against direct soundincoming from the dash panel 10, that is, the enhancement of thetransmission loss in the medium to high frequency domain having lesstransmission loss, is attained by forming the air-impermeable resonancelayer 3 of a significantly less area-weight than that of the dash panel10 as a surface layer and interposing the sound absorption layer 2 ofthe air-flow resistance between the dash panel 10 and theair-impermeable resonance layer 3. Unlike the prior art technique, thetechnique of this embodiment regulates the interface between theair-impermeable resonance layer 3 and the sound absorption layer 2 (thatis, the adhesive force of the adhesive layer 4). The air-impermeableresonance layer 3 has the significantly reduced area-weight to be notgreater than 200 g/m². This induces transmission resonances in a lowfrequency domain as well as in a high frequency domain (shown as (1) inFIGS. 5( a) and 5(b)). The two-layer structure of this embodimenteffectively enhances the transmission loss (shown as (3) in FIG. 5( a)).

The structure of this embodiment ensures sufficient sound absorptioneven in an actual odd-shaped product having the sound absorption layer 2of varying wall thickness. The structure of the embodiment effectivelytakes advantage of the membrane resonance of the sound absorption layer2 and the air-impermeable resonance layer 3 to assure the high soundabsorption rate in the medium to high frequency domain, even when thethickness of the sound absorption layer 2 is reduced for attachment ofthe resulting product in the restricted space. When the air-impermeableresonance layer 3 has an area-weight of 50 g/m², the relation betweenthe thickness of the sound absorption layer 2 and the resonancefrequency fr is given as Table 1:

TABLE 1 Thickness of Sound Absorption Layer (mm) 30 25 20 10 5 Resonance1531 1677 2166 2652 3750 Frequency (Hz)

Sound enters the vehicle interior in a diffusing manner. Theair-impermeable resonance layer 3 is light in weight and has a lowrigidity, so that resonance arises independently in a narrow area. Theresonance frequency varies in a range of 1531 to 3750 Hz with avariation in thickness L of the sound absorption layer 2 in a range of30 to 5 mm. As shown in FIGS. 6( a) and 6(b), the presence of theair-impermeable resonance layer 3 assures the sufficient high soundabsorption rate in wide frequency domain, compared with the structurewithout any air-impermeable resonance layer.

A general spring-mass vibration model can be applied in this case. If weassume a mechanical spring including an air spring of the soundabsorption layer 2 and the total mass of the sound absorption layer 2and the air-impermeable resonance layer 3, a resonance frequency fr (Hz)is expressed by Equation 2 given below. In Equation 2, the springconstant k in the standard spring vibration equation is defined ask=ρ·C²/L, where ρ, C, m, and L respectively denote the air density (1.2kg/m³), the sound velocity (340 m/s), the area-weight (g/m²) of theair-impermeable resonance layer 3, and the thickness (mm) of the soundabsorption layer 2.

$\begin{matrix}{{f_{r} = {\frac{1}{2\pi}\sqrt{\frac{\rho\; C^{2}}{mL}}}}\begin{matrix}{{fr}\text{:}\mspace{14mu}{Resonance}\mspace{14mu}{frequency}} \\{\rho\text{:}\mspace{14mu}{Air}\mspace{14mu}{density}\mspace{14mu} 1.2\mspace{14mu}{kg}\text{/}m^{3}} \\{C\text{:}\mspace{14mu}{Sound}\mspace{14mu}{velocity}\mspace{14mu} 340\mspace{14mu} m\text{/}s} \\{m\text{:}\mspace{14mu}\begin{matrix}{{Area}\text{-}{weight}\mspace{14mu}{of}\mspace{14mu}{resonance}} \\{layer}\end{matrix}} \\{L\text{:}\mspace{14mu}\begin{matrix}{{Thickness}\mspace{14mu}{of}\mspace{14mu}{sound}} \\{{absorption}\mspace{14mu}{layer}}\end{matrix}}\end{matrix}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

The structure of this embodiment effectively enhances the soundabsorption rate in a frequency band of 250 to 500 Hz, which hasgenerally poor sound absorbing power. Sufficient adhesion of theair-impermeable resonance layer 3 to the sound absorption layer 2 addsthe mass of the sound absorption layer 2 to the spring-mass vibrationsystem and shifts the resonance frequency of the air-impermeableresonance layer 3 to a higher frequency. The adhesion also causes aresonance frequency to appear at a lower frequency (shown as (4) inFIGS. 7( a) and 7(b)). The restricting force of the sound absorptionlayer 2 decreases the reduction of the transmission loss due to theresonance (shown as (5) in FIGS. 7( a) and 7(b)). The spring-massincluding the air spring of the sound absorption layer 2 and the totalmass of the sound absorption layer 2 and the air-impermeable resonancelayer 3 induces resonance in a frequency band of 315 to 630 Hz and thusenhances the sound absorption rate in this frequency band (shown as (6)in FIGS. 7( a) and 7(b)).

The double-wall effect of the dash silencer 1 and the iron dash panel 10attain the greater transmission loss than that expected by the weightlaw. The significant reduction in weight of the surface layer (theair-impermeable resonance layer 3) desirably shifts the frequency oftransmission resonance, which worsens the double-wall effect, to thehigh frequency domain having the sufficiently high transmission loss.The extremely light-weight surface layer (the air-impermeable resonancelayer 3) and regulation of the adhesive force between theair-impermeable resonance layer 3 and the sound absorption layer 2 toensure the sufficient adhesive force and adhesion area decrease thereduction of the transmission loss due to the transmission resonance bythe damping property of the sound absorption layer 2 (see FIG. 7( a).The extreme lightness of the air-impermeable resonance layer 3 and theregulated thickness of the sound absorption layer 2 to be not greaterthan 50 mm well control the resonance frequency in a frequency band of315 to 4000 Hz, thus attaining the high sound absorption rate. Theair-impermeable resonance layer 3 alone causes resonance in a mediumfrequency domain (640 to 1250 Hz) that belongs to higher frequencydomain. Adhesion of the air-impermeable resonance layer 3 to the soundabsorption layer 2 with the sufficient adhesive force and adhesion area,on the other hand, causes resonance in the spring-mass vibration system,which utilizes the partial mass of the sound absorption layer 2, in alower frequency domain of 315 to 630 Hz. This effectively enhances thesound absorbing power (see FIG. 7( b)). The air-impermeable resonancelayer 3 of the dash silencer 1 of the first embodiment has thesignificantly less area-weight than the surface layer of the prior artstructure, but sufficiently insulates direct noise and sound incomingfrom the dash panel 10 (from the engine room) and effectively absorbsindirect noise and sound that incomes from another site (from the siteother than the engine room) and is reflected in the vehicle interior.

In the structure of the first embodiment, due to the flexibility andthinness of the air-impermeable resonance layer 3 and so on, sound andnoise in the vehicle interior interfere with this air-impermeableresonance layer 3, and the thin-membrane vibration of the soundabsorption layer 2 and the air-impermeable resonance layer 3 occurs.This absorbs sound at the interface between the air-impermeableresonance layer 3 and the sound absorption layer 2 by resonancephenomenon. Utilization of the adhesive layer 4 interposed between theair-impermeable resonance layer 3 and the sound absorption layer 2effectively control the frequency of sound absorbed at the interface.The structure of the first embodiment especially has the high soundabsorbing power in a frequency band of 1000 to 1600 Hz, which desirablyimproves the clarity of conversation. The area-weight of theair-impermeable resonance layer 3 of 10 to 500 g/m² and the varyingthickness of the sound absorption layer 2 in a range of 1 to 50 mmeffectively enhance the sheet resonance-based sound absorbing power inthe above frequency band, thus assuring the favorable stillness in thevehicle interior. Even when the thickness of the dash silencer 1 isreduced, the structure of the embodiment takes advantage of theresonance phenomenon of the sheets and thereby assures the high soundabsorption rate. Compared with the prior art sound insulator, thestructure of this embodiment significantly reduces the weight of theair-impermeable resonance layer.

Example 1 and Comparative Example

The graphs of FIGS. 8 and 9 show comparison of the transmission loss andthe sound absorption rate between Example 1 and Comparative Example.Example 1 and Comparative Example had the same structure, except theadhesion area of the adhesive layer 4 was 90% in Example 1 and was 20%in Comparative Example. The total thickness of the dash silencer 1 was22 mm; the respective thicknesses of the sound absorption layer 2, theair-impermeable resonance layer 3, and the adhesive layer 4 were 20 mm,2 mm, and 50 μm. The air-impermeable resonance layer 3 of the dashsilencer 1 was made of polypropylene foam (PPF: 30-fold expansion) andhad a specific gravity of 0.031 g/cm³, a thickness of 2 mm and anarea-weight of 62 g/m². The sound absorption layer 2 was made ofthermoplastic felt (conventional felt of synthetic polyester fibers andcrude cotton) and had a specific gravity of 0.06 g/cm³, a thickness of20 mm and an area-weight of 1200 g/m². The adhesive area of the adhesivelayer was 90%. The dash silencer 1 of Example 1 was manufactured byapplying an aqueous EVA adhesive onto the air-impermeable resonancelayer 3 of polypropylene foam (the 30-fold expansion and the thicknessof 2 mm) at a density of 50 g/m², and compressing the adhesive-appliedair-impermeable resonance layer 3 with the sound absorption layer 2 ofthermoplastic felt or needle-punched felt under a pressure of 1 kg/cm²for 60 seconds. When the adhesive is not readily dried, the compressiontime may be reduced to approximately 30 seconds under application ofheat. The adhesion strength is 2 to 8 N/25 mm and the adhesion area isapproximately 90% of the whole interface. The observed peeling state wassurface destruction of the thermoplastic felt of the sound absorptionlayer 2. Needle-punched felt has the higher resistance to surfacedestruction and gives the adhesion strength of 5 to 10 N/25 mm.

As shown in the graph of FIG. 8, the transmission loss in a frequencydomain of not lower than 400 Hz is higher in Example 1 having theadhesion area of 90% than in Comparative Example having the adhesionarea of 20%. The structure of Example 1 thus more effectively reducesnoise from the vehicle exterior into the vehicle interior. As mentionedabove, the needle-punched felt has the higher resistance to surfacedestruction and gives the adhesion strength of 5 to 10 N/25 mm.Selection of the needle-punched felt for the sound absorption layer 2thus further enhances the transmission loss by 1 to 3 dB in thisfrequency domain of not lower than 400 Hz, although not specificallyshown in the graph.

As shown in the graph of FIG. 9, the sound absorption rate in Example 1having the adhesion area of 90% has a slight decrease in a frequencydomain of 630 to 1600 Hz, compared with that of Comparative Examplehaving the adhesive area of 20%. This slight decrease is ascribed torestriction of the air-impermeable resonance layer by the adhesive forceand the adhesion area and the resulting vibration isolation and damping.The sound absorption rate is, however, still not lower than 0.6 (60%)and is sufficient for absorption of noise in the vehicle interior.Comparative Example has the higher sound absorption owing to theair-impermeable resonance layer. The structure of Example 1 having theadhesive area of 90% has the higher sound absorption rate in frequencybands other than the domain of 630 to 1600 Hz than the structure ofComparative Example having the adhesive area of 20%, owing to theresonance phenomenon of the air-impermeable resonance layer and thesound absorption layer related to the adhesive force and the adhesivearea. Example 1 thus more effectively reduces noise in these otherfrequency bands in the vehicle interior, compared with ComparativeExample. In addition, in the frequency domain of about 400 to 500 Hz, asound absorption rate of 0.7 (70%) is attained due to the resonancefrequency related to both of the air-impermeable resonance layer and thesound absorption layer. This effectively reduces noise in a mediumfrequency domain in the vehicle interior.

Second Embodiment

FIG. 10( a) shows a dash silencer 201 of a second embodiment. The dashsilencer 201 of the second embodiment has a similar structure to that ofthe dash silencer 1 of the first embodiment, so the explanation aboutthe first embodiment can be applied hereto. The primary difference isthat a sound absorption layer 202 of the dash silencer 201 has ahigh-density sound absorption layer 202 a and a low-density soundabsorption layer 202 b having different densities. The high-density andlow-density sound absorption layers 202 a and 202 b are arranged on theside of the dash panel 10, whereas an air-impermeable resonance layer203 is arranged on the side of the vehicle interior. The low-densitysound absorption layer 202 b is bonded to the dash panel 10.

One face of the high-density sound absorption layer 202 a is bonded tothe air-impermeable resonance layer 203 via an adhesive layer 204. Thehigh-density sound absorption layer 202 a has a density in a range of0.05 to 0.20 g/cm³ and a thickness in a range of 2 to 30 mm. Thelow-density sound absorption layer 202 b has a density in a range of0.01 to 0.10 g/cm³ and a thickness in a range of 2 to 30 mm, and isbonded to the other face of the high-density sound absorption layer 202a, which is opposite to the air-impermeable resonance layer 203, via anadhesive layer 202 c. The high-density sound absorption layer 202 a hasan initial compression repulsive force in a range of 30 to 400 N, whilethe low-density sound absorption layer 202 b has an initial compressionrepulsive force in a range of 0.5 to 200 N. The initial compressionrepulsive force of the high-density sound absorption layer 202 a is atleast 1.2 to 40 times the initial compression repulsive force of thelow-density sound absorption layer 202 b. The thickness of thehigh-density sound absorption layer 202 a occupies 20 to 80% of thethickness of the sound absorption layer 202. More preferably, theinitial compression repulsive forces of the high-density soundabsorption layer 202 a and the low-density sound absorption layer 202 bare respectively in a range of 200 to 300 N and in a range of 50 to100N, and the initial compression repulsive force of the high-densitysound absorption layer 202 a is at least 1.5 to 5 times the initialcompression repulsive force of the low-density sound absorption layer202 b, and the thickness of the high-density sound absorption layer 202a occupies 40 to 60% of the thickness of the sound absorption layer 202.

The high-density sound absorption layer 202 a and the low-density soundabsorption layer 202 b of the sound absorption layer 202 may form amulti-layer structure of two different materials or may otherwise form amono-layer structure of an identical material having a density gradientfrom a higher density to a lower density.

The sound absorption layer 202, the air-impermeable resonance layer 203,and the adhesive layer 204 are made of the same materials as those ofthe first embodiment.

The graph of FIG. 11 shows frequency-transmission loss curves withregard to the sound absorption layer 202 of the varying-density,two-layer structure including the high-density sound absorption layer202 a and the low-density sound absorption layer 202 b and with regardto a sound absorption layer of a fixed density in the dash silencer 201of the second embodiment shown in FIG. 10( a). The varying-density,two-layer structure of the sound absorption layer 202 including thehigh-density sound absorption layer 202 a and the low-density soundabsorption layer 202 b significantly enhances the transmission loss in amedium (640 to 1250 Hz) and higher frequency domain.

The graph of FIG. 12 shows frequency-sound absorption rate curves withregard to the sound absorption layer 202 of the varying-densitytwo-layer structure including the high-density sound absorption layer202 a and the low-density sound absorption layer 202 b in the dashsilencer 201 of the second embodiment shown in FIG. 10( a) and withregard to the sound absorption layer 2 of a fixed density in the dashsilencer 1 of the first embodiment shown in FIG. 3. The dash silencer 1of the first embodiment having the sound absorption layer 2 of the fixeddensity has a significantly increase in sound absorption rate only inthe medium frequency domain of 640 to 1250 Hz. The dash silencer 201 ofthe second embodiment having the sound absorption layer 202 of thedifferent densities, on the other hand, has an increase in soundabsorption rate not only in the medium frequency domain of 640 to 1250Hz but in a wide frequency domain of 315 to 4000 Hz. The dash silencer201 of the second embodiment having the sound absorption layer 202 ofthe different densities has the lower sound absorption rate in afrequency domain of 400 to 1600 Hz than the dash silencer 1 of the firstembodiment having the sound absorption layer 2 of the fixed density. Thestructure of the second embodiment has apparent peaks corresponding toresonance frequencies. This is ascribed to the rigidity of theair-impermeable resonance layer 203 and the high-density soundabsorption layer 202 a via the adhesive layer 204. The resonancefrequency is shifted to the higher frequency with an increase inrigidity. In the structure of the second embodiment, a peakcorresponding to a lower resonance frequency also appears in a lowfrequency domain of 125 to 500 Hz. This is not affected by a variationin rigidity with the varying density of the sound absorption layer 202,but is ascribed to the functions according to a spring of the soundabsorption layer 202 and the total mass of the air-impermeable resonancelayer 203 and the sound absorption layer 202 in the spring-massvibration system.

The graph of FIG. 13 shows frequency-sound absorption rate curves in thedash silencer 201 of the second embodiment shown in FIG. 10( a) havingthe sound absorption layer 202 of the varying-density, two-layerstructure and the adhesive layer 204 with regard to varying masses ofthe air-impermeable resonance layer 203. The graph of FIG. 13 is basedon the data in the absence of the second sound absorption layer 306. Inthe graph of FIG. 13, the resonance frequency of the sound absorptionrate appearing as a peak in the higher frequency domain varies with avariation in mass of the air-impermeable resonance layer 203. Thisphenomenon is observed, regardless of the presence or the absence of asecond sound absorption layer 306. The graph of FIG. 13 is thusapplicable to the cases where the second sound absorption layer 306exists as well as to the cases where the second sound absorption layer306 does not exist. The resonance frequency in the higher frequencydomain varies according to the mass of the air-impermeable resonancelayer 203. There is a peak at a resonance frequency of 1250 Hz when themass of the air-impermeable resonance layer 203 is 60 g/m². Thiscorresponds to a thickness of 2 to 3 mm in the case of the foam and to athickness of 20 to 100 μm in the case of the film. The resonancefrequency is shifted to 1000 Hz when the mass of the air-impermeableresonance layer 203 is 300 g/m², and is shifted to 315 Hz when the massof the air-impermeable resonance layer 203 is 2000 g/m². The significantincrease in mass of the air-impermeable resonance layer 203 undesirablyshifts the resonance frequency to the lower frequency domain and causesinsufficient sound absorption in a desired frequency domain.

In the ultra-light dash silencer 201 of the second embodiment shown inFIG. 10( a) including the varying-density sound absorption layer 202 andthe air-impermeable resonance layer 203 via the adhesive layer 204,there is a vibration, due to the air spring of the sound absorptionlayer 202 and the total mass of the air-impermeable resonance layer 203and the sound absorption layer 202. There is also another vibration, dueto the spring of the air spring of the sound absorption layer 202 andthe rigidity of the air-impermeable resonance layer 203, and the mass ofthe air-impermeable resonance layer 203. The vibration due to the airspring of the sound absorption layer 202 and the total mass of theair-impermeable resonance layer 203 and the sound absorption layer 202gives a peak of sound absorption rate in the low frequency domain of 125to 500 Hz in the graph of FIG. 15 without the second sound absorptionlayer 306 (discussed later). The vibration due to the spring of the airspring of the sound absorption layer 202 and the rigidity of theair-impermeable resonance layer 203, and the mass of the air-impermeableresonance layer 203 gives another peak of sound absorption rate in ahigh frequency domain of 1600 to 6400 Hz in the graph of FIG. 15 withoutthe second sound absorption layer 306. The peak of sound absorption ratein the high frequency domain is affected by the coincidence effectrelating to the rigidity of the air-impermeable resonance layer 203bonded to the high-density sound absorption layer 202 a via the adhesivelayer 204.

Example 2

The structure of Example 2 was similar to the structure of Example 1,except the varying-density of the sound absorption layer. Thehigh-density sound absorption layer 202 a was made of thermoplastic felt(of reused synthetic fibers and PE fibers with PET used as bindingfibers) and had a density of 0.100 g/cm³, a thickness of 10 mm, anarea-weight of 1000 g/cm², and an initial compression repulsive force of200 N. The low density sound absorption layer 202 b was made of cottonfiber felt and had a density of 0.04 g/cm³, a thickness of 10 mm, anarea-weight of 400 g/m², and an initial compression repulsive force of50 N. The adhesive force of the adhesive layer 204 was 5 N/25 mm. Thehigh-density sound absorption layer 202 a and the low-density soundabsorption layer 202 b may be made of PET felt and joined together byneedle punching.

Third Embodiment

A dash silencer 301 of a third embodiment shown in FIG. 10( b) has avehicle interior-side adhesive layer 305 and a second sound absorptionlayer 306, in addition to the structure of the dash silencer 201 of thesecond embodiment shown in FIG. 10( a). In the dash silencer 301 of thethird embodiment, an air-impermeable resonance layer 303 is bonded tothe light-weight second sound absorption layer 306 via the vehicleinterior-side adhesive layer 305 having an arbitrary thickness, forexample, a thickness of 20 to 100 μm. The second sound absorption layer306 has a density in a range of 0.01 to 0.1 g/cm³ or more preferably ina range of 0.02 to 0.04 g/cm³ and a thickness in a range of 1 to 10 mmor more preferably in a range of 4 to 6 mm.

In the dash silencer 301 of the third embodiment, the second soundabsorption layer 306 is added to improve the sound absorption in thehigh frequency domain in the vehicle interior. The graphs of FIGS. 14 to16 shows the effects of the second sound absorption layer 306 and theadhesive conditions onto the air-impermeable resonance layer 303. Thegraph of FIG. 14 shows the effects of the second sound absorption layer306 onto transmission loss. The graphs of FIGS. 15 and 16 show theeffects of the second sound absorption layer 306 onto the soundabsorption rate. As shown in the graph of FIG. 14, the structure (1)without the second sound absorption layer 306 and the structure (2) withthe second sound absorption layer 306 under the condition of dotadhesion at a pitch of 100 mm have the better settings of transmissionloss than the structure (3) with the second sound absorption layer 306under the condition of adhesion at 10 N/25 mm. The structure (1) has alittle higher transmission loss than the structure (2). The graph ofFIG. 15 shows the sound absorption rate of the structure without thesecond sound absorption layer 306. The air-impermeable resonance layer303 vibrates sympathetically under the condition of a less restriction.Peaks of sound absorption rate corresponding to resonance frequenciesappear in the high frequency domain of 1600 to 6400 Hz and the lowfrequency domain of 125 to 500 Hz. Restriction of the air-impermeableresonance layer 303 with a material of no sound absorbing power lowersthe sound absorption rate in the high frequency domain of 1600 to 6400Hz as shown by an open arrow. The second sound absorption layer 306formed on the air-impermeable resonance layer 303 restricts theair-impermeable resonance layer 303 which is the surface layer. As shownin the graph of FIG. 16, while the presence of the second soundabsorption layer 306 lowers the peak of sound absorption rate in thehigh frequency domain, the additional sound absorbing power of thesecond sound absorption layer 306 effectively heightens the resultingsound absorption rate in a medium (640 to 1250 Hz) to high (1600 to 6400Hz) frequency domain, compared with the structure under restriction ofthe air-impermeable resonance layer 303 with a material of no soundabsorbing power.

In the ultra-light dash silencer 301 of the third embodiment shown inFIG. 10( b) including the vehicle interior-side adhesive layer 305 andthe second sound absorption layer 306, there is a vibration, due to theair spring of a first sound absorption layer 302 and the total mass ofthe second sound absorption layer 306, the air-impermeable resonancelayer 303, and the first sound absorption layer 302. This gives a peakof sound absorption rate in the low frequency domain of 125 to 500 Hz inthe graph of FIG. 16 with the second sound absorption layer 306. Thereis also another vibration, due to the air spring of the first soundabsorption layer 302 and the mass of the second sound absorption layer306 and the air-impermeable resonance layer 303. This gives another peakof sound absorption rate in the high frequency domain of 1600 to 6400 Hzin the graph of FIG. 16 with the second sound absorption layer 306restricting the air-impermeable resonance layer 303. This model is alsoaffected by the coincidence effects.

The varying density of the sound absorption layer 302 affects thecoincidence effects of a high-density sound absorption layer 302 a andthus affects a peak of sound absorption rate in the high frequencydomain.

Example 3

The structure of Example 3 had the second sound absorption layer 306bonded to the air-impermeable resonance layer 303 by dot adhesion at apitch of 100 mm, in addition to the structure of Example 2. The secondsound absorption layer 306 was made of thermoplastic felt (of reusedsynthetic fibers and PE fibers with PET used as binding fibers) and hada density of 0.04 g/cm³, a thickness of 5 mm, an area-weight of 200g/cm², and an initial compression repulsive force of 50 N.

Fourth Embodiment

A dash silencer 401 of a fourth embodiment is discussed below withreference to FIG. 17. The dash silencer 401 of the fourth embodiment hasa mono-layer first sound absorption layer 402 of a fixed density (or amulti-layer first sound absorption layer of a fixed density), in placeof the first sound absorption layer 302 in the dash silencer 301 of thethird embodiment, and otherwise has the similar structure to that of thedash silencer 301 of the third embodiment. Parts in the force embodimentwere numbered, adding 400 to the number of the corresponding part in thethird embodiment. The similar constituents are not specificallydescribed here. In the dash silencer 401 of the fourth embodiment shownin FIG. 17, the vehicle interior, a second sound absorption layer 406,an adhesive layer 405, an air-impermeable resonance layer 403, anotheradhesive layer 404, the first sound absorption layer 402, and thevehicle exterior (for example, an engine room) are arranged in thisorder. The first sound absorption layer 402 is fixed to the dash panel10 as the vehicle body, and the second sound absorption layer 406 facesto the vehicle interior. A modified structure of the dash silencer 401without the adhesive layer 404 gives a spring-mass single vibrationmodel including the air-impermeable resonance layer 403 as the mass andthe first sound absorption layer 402 as the spring. Namely the simplemembrane resonance of the air-impermeable resonance layer 403 arises inthe medium frequency domain of 640 to 1250 Hz. In the structure of thedash silencer 401 with the adhesive layer 404, on the other hand,resonance arises in the low frequency domain of 125 to 500 Hz,simultaneously with the membrane resonance of the air-impermeableresonance layer 403 in the medium frequency domain of 640 to 1250 Hz.This shows a spring-mass vibration system including the spring of thefirst sound absorption layer 402 and the mass of the air-impermeableresonance layer 403 and the first sound absorption layer 402.

The graph of FIG. 18 shows the effects of the adhesive layer 404 on thetransmission loss. As clearly shown in the graph of FIG. 18, thepresence of the adhesive layer 404 effectively enhances the transmissionloss in the low frequency domain of 125 to 500 Hz. FIG. 19 shows theeffects of the adhesive layer 404 on the sound absorption rate. As shownin the graph of FIG. 19, the structure without the adhesive layer 404gives an extreme increase in sound absorption rate only in the mediumfrequency domain of 640 to 1250 Hz. The structure with the adhesivelayer 404, on the other hand, gives an increase in sound absorption ratein a wide frequency domain including the low frequency domain of 125 to500 Hz and the high frequency domain of 1600 to 6400 Hz, as well as themedium frequency domain of 640 to 1250 Hz. The principle is that, in thestructure without the adhesive layer 404, the air-impermeable resonancelayer 403 alone causes resonance in the medium frequency domain of 640to 1250 Hz. In the structure with the adhesive layer 404, on the otherhand, resonance arises in the low frequency domain of 125 to 500 Hz,simultaneously with the resonance in the medium frequency domain of 640to 1250 Hz.

Example 4

The structure of Example 4 had the mono-layer first sound absorptionlayer 402, in place of the multi-layer first sound absorption layer 302of Example 3. The first sound absorption layer 402 was made ofthermoplastic felt (of reused synthetic fibers and PE fibers with PETused as binding fibers) and had a density of 0.04 g/cm³, a thickness of5 mm, an area-weight of 200 g/m², and an initial compression repulsiveforce of 50 N.

Fifth Embodiment

A floor silencer 501 of a fifth embodiment shown in FIG. 20 is fixed toan iron floor panel 510, which parts the vehicle interior from thevehicle exterior, and is arranged along the inner wall of the vehicleinterior. The floor silencer 501 is designed to be ultra light in weightfor the enhanced fuel efficiency and the easy attachment but to stillhave sufficient sound absorption properties. In the floor silencer 501of the fifth embodiment, the vehicle interior, a surface/backing layer507, a multi-layer second sound absorption layer 506, an air-impermeableresonance layer 503, an adhesive layer 504, a sound absorption layer502, a floor panel 510 as the vehicle body, and the vehicle exterior arearranged in this order. The sound absorption layer 502 is bonded to thefloor panel 510, and the air-impermeable resonance layer 503 is locatedon the side of the vehicle interior.

The floor silencer 501 of the fifth embodiment has partly identicalphysical properties with those of the dash silencer 401 of the fourthembodiment. Only different physical properties are given here. The soundabsorption layer 502 has a thickness in a range of 5 to 100 mm. Theair-impermeable resonance layer 503 has an area-weight of not greaterthan 600 g/m² or more preferably of not greater than 300 g/m². Theair-impermeable resonance layer 503 has a thickness in a range of 10 to600 nm or more preferably in a range of 20 to 300 nm in the case offilm. The second sound absorption layer 506 has a density in a range of0.01 to 0.2 g/m³ or more preferably in a range of 0.05 to 0.15 g/cm².

The surface/backing layer 507 is made of a surface material and abacking material, for example, polyethylene, EVA (ethylene vinyl acetatecopolymer), or SBR (styrene-butadiene copolymer rubber). The secondsound absorption layer 506 may have either a mono-layer structure or amulti-layer structure. In the illustrated structure of FIG. 20, thesecond sound absorption layer 506 has a two-layer structure including anupper layer 506 a and a lower layer 506 b.

The top face of the upper layer 506 a is bonded to the surface/backinglayer 507 via the adhesive layer 508. The bottom face of the upper layer506 a is bonded to or simply laid on the lower layer 506 b. The lowerlayer 506 b is made of hard sheet produced by compression of felt. Thebottom face of the lower layer 506 b is bonded to the air-impermeableresonance layer 503. The upper layer 506 a is made of a sound absorbingmaterial to enhance the sound absorption in the high frequency domain,while utilizing the elastic resonance of the lower layer 506 b toenhance the sound absorption in the high frequency domain. The lowerlayer 506 b and the sound absorption layer 502 utilize the rigidresonance and the elastic resonance of the lower layer 506 b to enhancethe sound absorption respectively in the medium frequency domain and inthe high frequency domain. The lower layer 506 b and the air-impermeableresonance layer 503 utilize the mass of the lower layer 506 b to enhancethe sound insulation.

Example 5-1

In the structure of Example 5-1 shown in FIG. 20, the surface/backinglayer 507 had an area-weight of 350 g/m², the upper layer 506 a was madeof felt and had a thickness of 5 to 15 mm, and the lower layer 506 b wasmade of hard sheet and had a thickness of 2 to 5 mm. The air-impermeableresonance film layer 503 had the thickness of 300 μm. The adhesive layer504 was made of an olefin adhesive material. The sound absorption layer502 was made of felt of blend of thermoplastic polyester, acryl, andcotton fibers or others and had a thickness of 10 mm. The structure ofExample 5⁻¹ also had an augmentation 509, for example, PP or PE beadfoam or RSPP compression molded object of 5 to 50 mm in thickness. Thefilmed hard sheet layer 506 b has an area-weight of 350 g/m².

FIG. 21( a) shows the structure of a floor silencer 501 a of ComparativeExample 1. The floor silencer 501 a of Comparative Example 1 has asurface/PE backing layer 507 d, a hard sheet layer 506 e, a felt layer503 f, and an augmentation layer 509 a. The surface/PE backing layer 507d, the hard sheet layer 506 e, and the felt layer 503 f are often bondedin advance to be integrated. The augmentation layer 509 a may beseparate from the other layers for the convenience of assembly of thevehicle. This structure of Comparative Example 1 hardly has soundabsorption effects in the vehicle interior, while the surface/PE backinglayer 507 d has some sound insulation effects to insulate noise incomingfrom the vehicle exterior.

FIG. 21( b) shows the structure of a floor silencer 501 b of ComparativeExample 2 according to the structure of FIG. 28. The floor silencer 501b of Comparative Example 2 has a surface/PE backing layer 507 g, a hardsheet layer 506 h, a felt layer 503 i, and an augmentation layer 509 b.This structure regulates the air permeation through the hard sheet layer506 h, thereby ensuring sound insulation of noise incoming from thevehicle exterior, as well as sound absorption in the vehicle interior.The air permeation, however, reduces the sound insulation effects. FIG.21( c) shows the structure of a floor silencer 501′ as Example of thefifth embodiment. The floor silencer 501′ as Example of the fifthembodiment has a surface/backing layer 507′, a hard sheet layer 506′, anair-impermeable resonance film layer 503′, an adhesive layer 504′, afelt layer 502′, and an augmentation layer 509′. The hard sheet layer506′ and the air-impermeable resonance film layer 503′ are almost whollybonded to each other. This structure also ensures sound insulation ofnoise incoming from the vehicle exterior, as well as sound absorption inthe vehicle interior. The structure of the fifth embodiment additionallyutilizes the elastic resonance and the rigid resonance to attain thebetter sound absorption rate and the higher sound insulation power ofthe air-impermeable resonance film layer 503′.

As shown in FIG. 22( a), the transmission loss of Example of the fifthembodiment is improved, compared with Comparative Examples 1 and 2,especially Comparative Example 2. As shown in FIG. 22( b) the soundabsorption rate of Example of the fifth embodiment is also improved,compared with Comparative Examples 1 and 2, especially ComparativeExample 1. This effect is ascribed to the presence of the film layer503.

In one modification of the fifth embodiment, the upper layer 506 a maybe a perforated air-impermeable film, which has a thickness of 30 to 400μm or more preferably of 200 μm and is made of an olefin like PE or PP,with the lower layer 506 b being made of felt, instead of the hardsheet. The upper layer 506 a and the lower layer 506 b are joined witheach other by needle punching. FIGS. 23( a) and 23(b) show the effectsof the perforated air-impermeable film. The presence of the perforatedair-impermeable film enhances both the transmission loss and the secondabsorption rate. The value of transmission loss of FIG. 23( a) wasdecided assuming the transmission loss of a 0.8 mm iron plate to be 0dB.

Example 5-2

In the structure of Example 5-2, the surface/backing layer 507 had anarea-weight of 350 g/m², the upper layer 506 a was made of anair-impermeable film and had a thickness of 200 μm, the hard sheet layer506 was a compression molded object of thermoplastic felt and had athickness of 5 mm, and the film layer 503 was made of PE film and had athickness of 300 μm. The adhesive layer 504 was made of an olefinadhesive, the felt layer 502 was made of thermoplastic felt of mainlypolyester fibers and had a thickness of 10 mm, and the augmentationlayer 509 was made of PP bead foam and had a thickness of 5 to 40 mm.The filmed hard sheet layer 506 had an area-weight of 350 g/m².

Sixth Embodiment

FIG. 24 shows a floor silencer 601 of a sixth embodiment. The structureof the floor silencer 601 of the sixth embodiment is similar to that ofthe floor silencer 501 of the fifth embodiment discussed above, exceptthat a sound absorption layer 602 includes a high-density soundabsorption layer 602 a and a low-density sound absorption layer 602 b.The high-density sound absorption layer 602 a and the low-density soundabsorption layer 602 b have partly identical physical properties withthose of the high-density sound absorption layer 302 a and thelow-density sound absorption layer 302 b of the dash silencer 301 of thethird embodiment shown in FIG. 10( b). Only different physicalproperties are given here. The high-density sound absorption layer 602 ahas a thickness in a range of 2 to 70 mm and an initial compressionrepulsive force in a range of 30 to 600 N or more preferably in a rangeof 50 to 300 N. The low-density sound absorption layer 602 b has athickness in a range of 2 to 70 mm and an initial compression repulsiveforce in a range of 5 to 300 N or more preferably in a range of 10 to100N.

Example 6

The structure of Example 6 had the high-density sound absorption layer602 a and the low-density sound absorption layer 602 b, in place of thesound absorption layer 502 of Example 5-1. The high-density soundabsorption layer 602 a was made of thermoplastic felt (of reusedsynthetic fibers and PE fibers with PET used as binding fibers) and hada density of 0.100 g/cm², a thickness of 10 mm, an area-weight of 1000g/m², and an initial compression repulsive force of 300 N. The lowdensity sound absorption layer 602 b was made of cotton fiber felt andhad a density of 0.04 g/cm², a thickness of 10 mm, an area-weight of 400g/m², and an initial compression repulsive force of 100 N. The adhesiveforce of the adhesive layer 604 was 5 N/25 mm. The high-density soundabsorption layer 602 a and the low-density sound absorption layer 602 bmay be made of PET felt and joined together by needle punching.

The air permeability is measured with a Frazil-type air permeabilitytester in conformity with JIS L1018 8.3.3.1 concerning air permeabilityof knitted fabrics or an equivalent air permeability tester havingextremely high correlativity.

Measurement of the transmission loss follows JIS A 1409. But the size ofeach sample was 1 m², instead of 10 m². FIG. 25 is a plan view showing ameasurement chamber of the transmission loss, where a speaker 20 andmicrophones 31 through 36 are set in the measurement chamber and eachsample, such as the dash silencer 1, is located on the wall of themeasurement chamber.

Measurement of the sound absorption rate follows JIS A 1416 (soundabsorption in a reverberation chamber). But the size of each sample was1 m², instead of 10 m². FIG. 26 is a plan view showing a measurementchamber of the sound absorption rate, where a speaker 40 and microphones51 through 53 are set in the measurement chamber and each sample, suchas the dash silencer 1, is located on the floor of the measurementchamber.

The embodiments and their examples discussed above are to be consideredin all aspects as illustrative and not restrictive. There may be manymodifications, changes, and alterations without departing from the scopeor spirit of the main characteristics of the present invention. Allchanges within the meaning and range of equivalency of the claims aretherefore intended to be embraced therein.

The scope and spirit of the present invention are indicated by theappended claims, rather than by the foregoing description.

What is claimed is:
 1. An ultra-light sound insulator, comprising: asound absorption layer that is light in weight and has a thickness in arange of 1 to 50 mm, the thickness varying from one region to another ina range not greater than 50 mm, and a density in a range of 0.01 to 0.2g/cm²; and an air-impermeable resonance layer in the form of a foam thatis bonded to said sound absorption layer via an adhesive layer and hasan area-weight of not greater than 200 g/m² and a thickness in a rangeof 1 to 7 mm, wherein an adhesion strength of said adhesive layeragainst said sound absorption layer and said air-impermeable resonancelayer is set in a range of 1 to 20 N/25 mm under conditions of a peelangle of 180 degrees and a peel width of 25 mm, an adhesion area of saidadhesive layer is 50 to 100% of a whole interface between said soundabsorption layer and said air-impermeable resonance layer so thatresonance due to a total mass of said air-impermeable resonance layerand said sound absorption layer occurs in addition to membrane resonanceof said air-impermeable resonance layer, and said sound absorption layeris adapted to face to a vehicle body panel, while said air-impermeableresonance layer is adapted to face to a vehicle interior.
 2. Anultra-light sound insulator in accordance with claim 1, wherein saidsound absorption layer has an initial compression repulsive force in arange of 2 to 200 N.
 3. An ultra-light sound insulator in accordancewith claim 1, wherein said sound absorption layer has a density in arange of 0.03 to 0.08 g/cm³.
 4. An ultra-light sound insulator inaccordance with claim 1, wherein said adhesion strength of said adhesivelayer against said sound absorption layer and said air-impermeableresonance layer is set in a range of 3 to 10 N/25 mm under conditions ofa peel angle of 180 degrees and a peel width of 25 mm.
 5. An ultra-lightsound insulator in accordance with claim 1, wherein said adhesion areaof said adhesive layer is 80 to 100% of a whole interface between saidsound absorption layer and said air-impermeable resonance layer.
 6. Anultra-light sound insulator in accordance with claim 1, wherein saidsound absorption layer has a density in a range of 0.03 to 0.08 g/cm³,said air-impermeable resonance layer has an area-weight of not greaterthan 200 g/m², said adhesion strength of said adhesive layer againstsaid sound absorption layer and said air-impermeable resonance layer isset in a range of 3 to 10 N/25 mm under conditions of a peel angle of180 degrees and a peel width of 25 mm and said adhesion area of saidadhesive layer is 80 to 100% of a whole interface between said soundabsorption layer and said air-impermeable resonance layer.
 7. Anultra-light sound insulator in accordance with claim 5, wherein saidsound absorption layer has an initial compression repulsive force in arange of 20 to 100N.
 8. An ultra-light sound insulator in accordancewith claim 1, wherein the thickness of said sound absorption layer is ina range of 5-40 mm.